Nymphs
Clown Bugs go through five nymph stages (instars), shedding their skin after each stage. There are various colour forms of the nymphs and the stage should be judged by the size of the wing buds. Only the adults have fully developed wings.
The following thumbnails link to a page with a larger picture.
First Instar
Second Instar
Third Instar
Fourth Instar
Fifth Instar
Brisbane Insects has some interesting observations on the clown bug.
Classification
from Australian Faunal Directory
Order Hemiptera - bugs, hoppers and scale insects
Suborder Heteroptera - true bugs, 2806 named species in Australia
Family COREIDAE - Crusader Bugs or Squash bugs, 92 species in Australia
Genus Amorbus
Species Amorbus robustus